Transmission of pictures by electricity



Nov. 13, 1928. 1,691,071

H. E. IVES ET AL TRANSMISSION OF PICTURES BY ELECTRICITY Filed May 14, 1924 2 Sheets-Sheet 1 Nov. 13, 1928.

H. E. IVES El AL TRANSMISSION OF PICTURES BY ELECTRICITY Filed May 14',

1924 2 Sheets-Sheet 2 29 I i Y Y 28' I Y hrs/70s.

' Patented Nov. 13,- 1928.

* UNITED STATES I g 1,691,071 PATENT OFFICE.

HERBERT E IVES, OF MONTCLAIR, AND MAURICE B. LONG, 0F GLEN RIDGE, NEW JER- I SEY, ASSIGNORS TOWESTERN ELECTRIC-COMPANY, INCORPORATED, 015 NEW YORK,

' Application filed May 14,

This invention relates to the transmission of pictures by electricity and more particu-' larly to a system having a linear transmission characteristic.

In a copending application of M. B. Long, Serial No. 681,347, filed December 18, 1923, and patent to E. C. WVeIite, N 0. 1,686,355, Oct. 2, 1928, systems for transmitting pictures by electricity are disclosed. In these systems, a voice frequency carrier wave is modulated by amplified current from a photoelectric cell which itself is variably excited by light intensities corresponding to the tone valves of elemental areas of the picture. The picture is reproduced'*-from the modulated wave by a light valve. In the Wente arrangement, the modulated Wave is impressed directly upon the movable conductors of the light valve. Unless suitable precautions are taken distortion may be introducedby the circuit elements and lack of-faithfulnessof reproduction will result.

.An object of tlllSlIlVGl'lt-lOIldS to obtain faithfulness of reproduction. Another object is to obtain this result-by operating each circuit arrangement linearly.

These objects areattained in one embodiment of the invention by operating each circuit' element within a finite limiting range between the extremities of which the characteristic'of the arrangement is linear. The urrent from a photo-electric cell is first am:

' plified by'a two-stageelectron discharge amplifier after which it is impressed upon an electron discharge modulator in order to modulate a. voice frequency carrier Wave.

After transmission to the reproducing terminal, the modulated. wave is amplified and impressed upon the movable conductor of asingle string light valve. The movement of the string causes the exposure of a light sen- I sit-ive record blank to reproduce the picture.-

The adjustment is such that the voltage im-" pressedupon the first stage of the amplifier is directly proportional to the light incident upon the photoelectric cell and the current upon the cell.

output from each stage of theamplifier and. from the gnqdulator is directly roportional to the voltage impressed -u on t 'e respective devices within the limits 0- the light incident This results in the carrier wave having a finite amplitude greater'than zero at both limits of its working range. The

light valve is provided-with adjustable jaws N. Y., A. CORPORATION OF NEW YORK.

1924 Serial No. 713,362.

do not limit this invention to the production of a picture record at the receiver from a picture record at the transmitter. The invention is equally applicable to systems, for example, in Which a transitory image is produced at the receiver, as in certain forms of television systems' The novel features which are considered characteristic of this inventidn are set forth with particularity in the appended claims.

The invention, both as to its organization and method of operation together with other ob-' jects and advantages thereof, will be further explained in the following description} having reference to the accompanying drawing con: sisting of the following figures.

Fig. 1 isa schematic circuit arrangement of a sending terminal according to this invention. J

Fig. 2 1s a schematlccircuit arrangement of a reproducing terminal to cooperate'with" the arrangement of Fig. 1. j Fig. 3 is a top view of the optical arrangement of Fig. 2 partially in cross section.

Figs. 4 and 5 are-fragmentary views of details of the light valve greatly enlarged. Fig. 6 shows characteristic curves'of the circuit elements ofthe sendin equipment.

. Fig. 7- illustrates schematica ly an arrangement for rotating the picture drum and simultaneously moving it axially.

A system according to this invention is illustrated b placing Figs. 1 and 2 side by side from le. to right, respectively. n 'Referring. to the drawing so arranged, a

sending drum 5 at the sending; terminal foa' carries a ,transparencyofthe pictur to be i transmitted; vA receiving drum 6 at he reproducing terminalfcarries a light sensitive record blank. The: picture drums 5 and 6 are driven synchronously and moved axially by a suitable synchronizing system such for example, as that disclosed in the Long appllcation supra. \Vithin the sending drum 5 a photoelectric cell 7 is located which is con nected to the input circuit of electron discharge device 8 by photoelectric cell battery 9 and coupling resistance. 10. The electron discharge device 8 with its associated input and output circuits forms the first stage of a two-stage picture current amplifier PCA. The second stage of this amplifier consists of electron discharge device 11. Device 11 is coupled to device 8 through battery 12 and coupling resistance 13. The current in the photoelectric cell 7 is controlled by light from source 14 directed by lens 15 through an aperture in shield 16 and the transparency of the picture on drum 5.

' An electron discharge modulator 'M is coupled to a source of voice frequency carrier waves CO by means of transformer 17 and resistance 18. The modulator M is also con nected to the output circuit of amplifier PCA through battery 19 and resistance 20. The output circuit of the modulator M is coupled to a transmission line L by means of transformer 21. At the receiving terminal, the modulated carrier waves from the line L are amplified in a two-stage electron discharge amplifier RA and impressed upon string 22 of light valve V. The amplifier RA consists of two electron discharge devices 23 and 24 with their associated input and output circuits. The coupling between the input circuitof device 23 and the line L consists of potentiometer 25 and transformer 26. The

coupling between the output circuit of device 24 and the light valve V is through transformer'27. The vibration of string 22 with respect. to aperture forming jaws 28 and 29 determines the amount of exposure of the record blank on drum 6in an axial direction.

I The aperture in shield 30 determines the amount of exposure in the direction of rotation of the record blank. o

The string 22, the jaws 28 and 29 and the screen 30 will hereinafter be designated as I the aperture forming members.v The jaws 28 and 29 define what will be called hereinafter the horizontal aperture. The aperture in screen 30 will be called the vertical aperture.

The aperture forming members are intensely and uniformly illuminated by light from source 31 passing through lens 32. The area illuminated is at least as large as the largest opening defined by jaws 28 and 29 and screen 30. An image of the aperture formed by the aperture forming members is projected upon the record blank on drum 6 by lens 33.

An arrangement forrotating the picture drums and simultaneously moving them axially is illustrated in Fig. 7. It will be described with reference to the sending drum 5, but it is to be understood that a similar arrangement may be used at the reproducing terminal. The drum 5 is mounted on a shaft 53, which is carried by carriage 51 which itself is movable with respect to the frame 50.

The shaft 52 rotates, but is fixed longitudinally with respect to frame 50. It has screw engagement, however, with carriage 51 which is moved along frame 50 by the rotation of shaft 52. Shaft 53 is driven by shaft 52 through gear wheels 54 and 55. Gear wheel 54 is slidably mounted on shaft 52, but rotates with it due to a key which slides in a keyway.

Referring again to Fig. 1, the potential im- 'by that part of the battery 12. which is connected into the input circuit of device 11 and the potential drop across resistance 13 due to current- Il flowing in resistance 13. The potential impressed upon the modulator M, exclusive of the potential of the carrier wave from source CO, is represented by E-ZB and its value is determined by that portion of battery 19 connected in the input circuit of modulator M and the potential drop across resistance 20 due to current I2 flowing in resistance 20.. The amplitude of the currenifiowing in the line L is represented by I- Referring now to Fig. 6, the characteristics of the elements of Fig. 1 are shown by curves A, B, C, D, E, F, and G.

Curve A shows the value of current I-() flowing in resistance lOcorresponding to dif-,

ance 10. Between these two limiting conditions and for some distance beyond the curve A is hnear.

Curve B shows the relation between the potential E'1 impressed upon the input circuit of amplifier 8 and the current IO flowing in the resistance 10. The point I; corresponding to the dark condition is dependent upon the potential of'battery 88 which is represented by the line Ob. The point 0 corresponding to the light condition, or the point a on curve A, is determined by the magnitude of the r7sistance 10. Thus the slope of the curve B is determined by the magnitude of resistance 10.

"Curve C shows the relation between the potential E1 impressed upon the input circuit of amplifier 8 and the current I1 flowing in the resistance 13. Pointd corresponds to the dark condition of curve A, or point 6 of curve B. Point 6 corresponds to the light condition, or the point a of curve A, or the point 0 of curve B. 7

Curve D shows the relation between the potential -E2 impressed upon the input circuit ofamplifier 11 and the current I1 flowing in the resistance 13. Point 7 corresponds to the dark condition of curve A and point 9 to the light condition. The slope of curve D is determined by the value of resistance 13 while the intersection with the abscissa at h is dependent upon the potential of battery 12 which is included in the input circuit of amplifier 11.

Curve E shows the relation between the potential E2 impressed upon the input circuit of amplifier 11 and the current I2 flowing in the resistance 20. The shape and slope of the curve E is dependent upon the physical characteristics of the. device 11 and the value of resistance 20. The points is and 'm correspond to the dark and light conditions respectively of curve A. I

Curve F shows the relation between the potential E-3 impressed upon the input circuit of modulator M, exclusive of the potential due to the carrier source CO, and the current I2 flowing in the resistance 20. The slope of curve F. is dependent upon the value of resistance 20 while its intersection with the abscissa at point p is dependent upon the potential of that part of battery 19 connected in the input circuit of the modulator M. The points t and 0?. correspond respectively to the dark and light conditions of curve A.

The curve G shows the relation between the input voltage E3 and the amplitude of the current IL flowing in the line L. The

' shape of this curve is dependent upon the physical characteristics of. the modulator" electron discharge device M and the ampli-' tude of the carrier wave from the source CO. The point 8 corresponds to thedark condition and the point r to the light condition or point a of curve A. Bet-ween the points 1" and 8, curve G is substantially linear. In order to operate only over the linear portion of curve G, it is necessary that the minimum amplitude of the carrier wave as delivered to the line under the light condition shall have a finite value greater than zero. Thus,

in order that the amplitude of the carriercurrent delivered to the line shall bear a linear relation to the light intensities incident upon the photoelectric cell 7, the adjustment of the circuit elements must be such that the minimum value of, carrier current has a finite value above zero. The modulator may, therefore, be considered to have an electrical bias.

WVhen this modulated carrier wave is impressed upon the string 22 of light valve V,

the string 22 vibrates with an amplitude corresponding to the amplitude of the impressed current. In order to prevent the exposure of the record blank due to the amplitude of carrier wave corresponding to the point 1" of curve G, t-he light valve is mechanically biased by means of adjustable jaws 28 and 29, as shown in Fig. 5.

The horizontal aperture formed by jaws 28 and 29 is so adjusted that the string 22 will need to move the space 40 (Fig. 5) before an aperture will be formed through which lightpan pass to expose the record blank. Thus, the bias at the sending terminal which is necessary in order to obtain linear modulation-is compen-,

, sated for by bias at the reproducing terminal.

a distance represented by These jaws This method of operatinga transmission system is especially important in a-commerclal system using electron discharge devices. Such devices, when operated with any physically practicable adjustment have decided non-linearcharacteristics both at low and a high values of impressed input potential.

These characteristics can bemade substantially linear only between certain finite limiting values, neither of which is zero. In a commercial system, however, it is highly desirable to be able to operate each element over the linear portion of its characteristic, since each element can then be independently adjusted, after which each element can. be associated with the other elements of the system with the assurance that'the combination will function to givev faithful reproduction. This is especially important where the several elements in the system may be Widely separated geographically.. j i v V'a-rious modifications of the specific embodiment of theinvention as hereinbefore described maybe made withoutdeparting from the scope of the invention, as defined by the mechanically biasing the responsive device to compensate for the electrical bias of the modulator.

2. In a system for the transmission of pietures by electricity, an electron discharge modulator for modulating a carrier wave in accordance with the signal, said modulator having a characteristic between signal voltage impressed upon its input circuit and amplitude of carrier wave in the output circuit which is linear between two finite values, each greater than zero, a source of signal current for impressing potentials upon the input circuit of said modulator for operating over the linear portion only of said characteristic, and a receiver upon which said modulated wave is impressed, including biasing means for limiting the effect of saidv receiver to the linear portion only.

In a system for the transmission of pietures by electricity, an electron discharge modulator for modulating a carrier wave in accordance with the signal, said modulator hav- 1 mg a characteristic between signal voltage impressed upon its input circuit and amplitude of carrier wave in the output circuit which is linear between two finite values, each greater than zero, a source of signal current for impressing potentials upon the input circuit of said modulator for operating over the linear portion only of said characteristic, and an clectro-optical arrangement for htaining light beams, the effect of which is proportional to the amplitude of the signal current. 4

4. In a system for the transmission ofpic-' tures by electricity, a photo-electric cell, an electron discharge amplifier for amplifying current from said cell, an electron discharge modulator for modulating a carrier wave in accordance with said amplified current, means to "ary the light incident upon said cell between twoifinite values to vary the current in said cell, means to cause the potent'ial impressed upon said modulator due to said amplified current to vary between two finite values, neither of which is zero, and a demodulator for said modulated carrier wave which gives no useful response for those amplitudes of carrier waves less than that corre spending to the smaller of the two limiting values, neither of which is zero,'and a light I valve actuated by saidmodulated wave and adjusted-to transmit no light when amplitudes of the carrier wave are lessthan that corresponding to the smaller of the two limiting values of the impressed potential.

6. In a system for the transmission ofpietures by electricity, a photo-electric cell, a

two-stage, electron discharge amplifier for amplifying current from said cell, an electron discharge modulator for modulating a carrier wave in accordance with said amplified current, means to vary. the light incident upon said cell from zero to afinite value, means to cause the potential impressed upon said modulator due to the amplified current corresponding to varying light intensities incident upon said photo-electric cell to vary between two finite values, neither of which is zero, and a light valve actuated by said modulated wave and adjusted to transmit no light when the amplitudes of the carrier wave are less than that corresponding to the light condition of the photo-electric cell.

7. In a system for the transmission of pic tures by electricity, a source of carrier waves, means to modulate carrier waves from said source to produce modulated carrier waves having amplitudes which vary between two finite limiting values, neither of which is zero, corresponding to variations in the tone values of elemental areas of a picture between black and white, and detecting means for producing from said modulated carrier waves a picture the tone values of the elemental areas of which vary between black and white corresponding to the original.

1 8. In a system for the transmission of pictures by electricity, a source of carrier waves, means to modulate carrier waves from said source to produce modulated carrier waves having amplitudes which vary between two finite limiting values, neither of which is zero, corresponding to variations in the tone values of elemental areas of a picture between black and white, and detecting means for producing from said modu ated carrier waves a picture the tone values of the elemental areas of which vary between black and white and have intermediate tone values.

9. In a system for the transmission of pietures by electricity, a source of carrier waves, means'to modulate carrier waves from said source to produce modulated carrier waves having amplitudes which vary linearly be-' tween two finite limiting values, neither of which is zero, corresponding to variations in the tone values ofelemental areas of a picture between black. and white, and detecting means for producing from. said modulated carrier waves a picture the tone values of the elemental areas of which vary between black and white and have intermediate tone values.

In witness whereof, we hereunto subscribe our names this 14th day of May, A. D. 1924.

HERBERT E. IVES. MAURICE B. LONG. 

